The invention relates to a planar optical waveguide. The waveguide comprises a nonmagnetic substrate with a first magnetooptical layer provided expitaxially on the substrate. A second magnetooptical layer is provided expitaxially on the first magnetooptical layer. Both magnetooptical layers consist of an iron garnet-based material.
The invention further relates to a process for manufacturing such an optical waveguide.
In optical communication by means of glass optical fibers, single mode waveguides are used as optical isolators or possibly also as optical circulators to protect the semiconductor laser diodes used therein from light reflected back from the coupled section. Such waveguides utilize the nonreciprocal properties of magnetooptical materials based of the Faraday effect. Planar optical single mode waveguides formed by layers of magnetooptical material provided epitaxially on a substrate must have a radiation-conducting layer with a thickness which corresponds to the dimensions of the optical fibers to be coupled; monomode optical fibers, for example, have core diameters in the region of 5 to 10 .mu.m, meaning that the thickness of the radiation-conducting layer of a monomode waveguide must also lie in the region of 5 to 10 .mu.m.
Layers with these dimensions can be made from highly diluted molten solutions via liquid phase epitaxy (LPE), where the solvent usually consists of a mixture of PbO and B.sub.2 O.sub.3. A suitable material for the magneto-optical layer is, for example, yttrium-iron garnet (Y.sub.3 Fe.sub.5 O.sub.12). For the substrate, on which such layers grow epitaxially, a suitable material is gadolinium-gallium garnet (Gd.sub.3 Ga.sub.5 O.sub.12) in the form of the 0.5 mm thick (111) single-crystal slice (which is commercially available). Other iron garnets are also suitable for the magnetooptical layers, such as, for example, Gd.sub.3 Fe.sub.5 O.sub.12 or bismuth-substituted iron garnets.
It is known to grow two iron garnet layers of different compositions consecutively on such single-crystal substrate slices in two separate epitaxial processes from two different melts. J. Pistora et al describe in an article entitled "Mode Spectroscopy of Double-Layer Magnetic Garnet Films" (IEEE Transactions on Magnetics, Vol. MAG-20, No. 5, pages 1057-1059, September, 1984) how a Sm- or Ga-substituted yttrium-iron garnet layer, 0.8 .mu.mm thick with a refractive index n.sub.1 =2.18.+-.0.02 is produced epitaxially from a melt on a substrate. They also describe how subsequently a second epitaxial layer with a thickness of 1.5 .mu.m and a refractive index n.sub.2 =2.30.+-.0.02 is produced from nominally unsubstituted yttrium-iron garnet.
The following disadvantages are connected with this known layer structure:
In optical communication by means of optical fibers, monomode transmission is used for obtaining a high data rate. Due to the wide difference in refractive index .DELTA.n=n.sub.2 -n.sub.1 =0.12 of the known layer structure, monomode data transmission is not possible; only multimode data transmission is possible.
Further disadvantages of the known waveguide are that two different melts have to be used for the manufacture of the magnetooptical layers, so that two separate working processes are therefore required. The setting of the differene in refractive index .DELTA.n between the two magnetooptical layers is highly inaccurate.
A further disadvantage of the known layer is that a transient layer is produced between the two iron garnet layers (Y.sub.3 (Fe,Ga).sub.5 O.sub.12 and Y.sub.3 Fe.sub.5 O.sub.12) as a result of the second epitaxial process. The transient layer has a different composition and consequently also a different refractive index.